The present invention relates to a low phase-noise voltage controlled oscillator (VCO), primarily to be realised as a VCO integrated on a single chip.
Voltage controlled oscillators are found in a number of applications. With the advance of mobile communications, there is a growing need for VCO-s, which must meet strict technical specifications. Simultaneously, there is also a constant demand for a high level of integration of the electronic components.
Communications equipment operating in the GHz range requires VCOs working in these frequencies. On lower frequencies, it is normally not possible to integrate VCO-s fully onto a single chip, because the quality factor (Q) of the inductors does not reach the necessary values. However, it is possible to manufacture integrated inductors for the GHz frequency range. It has been shown that the design of inductors operating between 1-5 GHz is feasible with existing integrated circuit technologies.
One of the critical parameters of VCOs used in communications equipment is the phase noise. Low phase noise is desirable to avoid the interference between neighbouring transmission channels, which operate on frequencies close to each other. Typically, for a VCO operating in the lower GHz range, it is required that the phase noise is below xe2x88x92100 dB/Hz at a 100 kHz offset from the carrier frequency.
It is known that the quality factor of a VCO may be improved with differential circuits. Particularly, the use of a cross-coupled transistor pair together with a passive resonator has been suggested by several authors. The passive resonator mostly consists of an inductor-capacitance pair, the ends of which are connected to the collectors of the cross-coupled transistors. Efforts were mostly directed towards improving the quality factor of the inductor in the resonator.
U.S. Pat. No. 6,081,167 to Kromat discloses a differential VCO with an inductor-capacitance pair, and a pair of cross-coupled transistors, acting as amplifiers. There is a capacitance between the ends of the resonator and the bases of the transistors. In order to reduce the phase noise, it is suggested to use CMOS transistors as current switches, while using bipolar transistors as current source transistors. However, this solution does not concern or suggest any impedance matching between the resonator and the amplifier part, in order to reduce the resultant phase noise.
In an exemplary embodiment of a first aspect of the invention, there is disclosed a variable frequency oscillator circuit comprising a resonator circuit part and an amplifier circuit part. The resonator circuit part has a first end and a second end, and it comprises a parallel connected inductor-capacitance pair. The amplifier circuit part comprises first and second transistors having a collector, base, and emitter. The collector of the first transistor is connected to the first end of the resonator circuit part, while the collector of the second transistor is connected to the second end of the resonator circuit part. The base of the first transistor is cross-connected to the second end of the resonator circuit part and to the collector of the second transistor. The base of the second transistor is cross-connected to the first end of the resonator circuit part and to the collector of the first transistor. The amplifier circuit part further comprises a first impedance matching element connected between the base of the second transistor and the first end of the resonator circuit part, and a second impedance matching element connected between the base of the first transistor and the second end of the resonator circuit part.
Preferably, the impedance matching element is an inductor. In particularly advantageous embodiment, the amplifier circuit part and the resonator circuit part are integrated on a common single chip.
In another aspect of the invention, there is proposed a method for improving the phase-noise performance of a variable frequency oscillator circuit, where the VCO comprises a passive resonator circuit part and an active amplifier circuit part operatively connected to the passive resonator circuit part. The method comprises the step of providing a noise-matched optimum source impedance for the active amplifier circuit part.